The present invention relates generally to the formation of a hole in a support structure, and more particularly to a method of and a tool for forming a hole for insertion and anchoring of a fastening element therein.
There are already known various fastening elements which have spreading portions for the purpose of anchoring the fastening element in the support structure. The fastening elements here under discussion are conventional and, generally speaking, such fastening element includes an expansion sleeve of generally cylindrical configuration prior to the insertion and anchoring of the fastening element in the hole of a support structure, and a screw accommodated in the sleeve, such sleeve having spreading portions which, when the screw is tightened, spread apart and anchor the sleeve and thus the screw in the support structure. The spreading of the spreading portions may be achieved either by the screw itself which may be threaded into a converging hole in the sleeve, or by an expander element mounted on the screw and being drawn between the spreading portions pushing them apart during the tightening of the screws. Such arrangements achieve satisfactory results provided that the material of the support structure is such as to permit at least partial yielding of the walls surrounding the end of the hole in the region of which the spreading portions of the sleeve are accommodated. In this event, the spreading portions of the sleeve deform the walls in such a region, thus in effect forming a V-shaped or a conical enlarged portion in the hole so that the spreading portions of the sleeve engage behind the material of the support structure. It will be understood that the anchoring action of the fastening element, that is the load which the fastening element is capable of supporting, is proportional to the degree to which the spreading portions of the sleeve are spread apart for any given material of the support structure. Besides the force available for tightening the screw, the most important factor determinative of the degree of spreading of the spreading portions is the material used for the spreading portions, particularly its rigidity. However, experience has shown that when the conventional fastening elements are to be accommodated and anchored in holes provided in relatively rigid support structures, such as masonry walls or concrete structures, the anchoring effect is rather unsatisfactory due to the fact that even when high grade alloyed steels are used for the fastening element, the spreading of the spreading portions of the sleeve is minute since the material of the support structure will yield only slightly with disproportionately increasing force being needed for tightening the screw of the fastening element and spreading apart the spreading portions thereof. Furthermore, even if an unlimited tightening force were available, with such high rigidity of the walls surrounding the end of the hole it would be the spreading portions of the sleeve which would become deformed rather than the walls surrounding the hole.